The present invention relates to an amplifier circuit using a feedback load.
FIG. 1 is a circuit diagram showing a conventional wide-band feedback type differential amplifier circuit (H. Hillbrand, J. Gruber and P. Russer, "Computer Aided Design of a 1 GHz Band-width Monolithic Integrated Amplifier", ESSCI-RC, 1977, pp. 122-124).
Referring to FIG. 1, reference numerals 1a and 1b denote input terminals for receiving input signals V.sub.in and V.sub.in ; 2a and 2b, output terminals for outputting output signals V.sub.out and V.sub.out ; 3, a high-potential terminal for receiving a potential V.sub.CC ; 4, a low-potential power supply terminal for receiving a potential V.sub.EE ; 5a and 5b, a pair of input differential transistors as a first pair of differential transistors; 6a and 6b, a pair of output differential transistors as a second pair of differential transistors; 7a and 7b, and 8a and 8b, load resistors having resistances R.sub.L1 and R.sub.L2, respectively; 9a and 9b, load capacitors having a capacitance C.sub.L ; and 10 and 11, first and second current supply circuits for supplying currents I.sub.1 and I.sub.2.
The circuit shown in FIG. 1 is constituted by the pair of input differential transistors 5a and 5b connected to the load resistors 7a and 8a, and 7b and 8b as loads, respectively, and the pair of output differential transistors 6a and 6b connected to the load resistors 8a and 8b as loads. As the characteristic feature of this circuit, the collectors of the transistors 6a and 6b are connected to their bases through the load resistors 7a and 7b, respectively. Collector signals of the transistors 6a and 6b are fed back to their bases in opposite phases through the load resistors 7a and 7b, in other words, feedback loads are used for the transistors 5a and 5b, thereby realizing a wide-band operation. If transconductances of the transistors 5a and 5b, and 6a and 6b are represented by gm1 and gm2, base resistances are represented by rb1 and rb2, input capacitances are presented by C.pi.1 and C.pi.2, and an angular frequency is represented by .omega., a voltage gain A.sub.V of this conventional circuit is given by: ##EQU1## where a is the time constant at the bases of the transistors 5a and 5b, b is the time constant at the bases of the transistors 6a and 6b, and c is the time constant at the collectors of the transistors 6a and 6b, and these constants are respectively given by: EQU a=C.pi.1.multidot.rb1 (2) ##EQU2## EQU c=C.sub.L .multidot.R.sub.L2 /(1+gm2.multidot.R.sub.L2) (4)
The gain of the circuit shown in FIG. 1 in a low-frequency mode is approximated by gm1.multidot.R.sub.L1 from equation (1). The gain in a high-frequency mode is decreased as a frequency increases in accordance with the time constants a, b, and c. However, as can be seen from equations (3) and (4), since the time constants of the bases and collectors of the transistors 6a and 6b can be reduced to 1/(1+gm2.multidot.R.sub.L2) by a feedback effect, a wide-band operation is allowed as compared to a conventional differential amplifier circuit. The time constant b given by equation (3) has an imaginary number component. The imaginary number component is increased as the capacitances C.sub.L of the load capacitors 9a and 9b are increased, and operates to cause peaking in frequency characteristics. For this reason, the capacitances C.sub.L of the load capacitors 9a and 9b are optimized, thus achieving a wider-band operation.
FIG. 2(a) is a graph showing simulation results of frequency characteristics of the circuit shown in FIG. 1. In FIG. 2(a), a frequency is plotted along the abscissa, and a gain is plotted along the ordinate. A plurality of characteristics 21, 22, 23, and 24 are obtained by changing the capacitance C.sub.L of the load capacitor to 0.2 pF, 0.1 pF, 0.05 pF, and 0. As can be seen from the graph, peaking characteristics can be provided by increasing the capacitance C.sub.L, and the capacitance C.sub.L is optimized to realize wide-band characteristics. For this reason, wide-band characteristics can be obtained by utilizing an input capacitance or wiring capacitance of the output-side circuit. Note that a bipolar transistor having a high-frequency cutoff frequency of 60 GHz was assumed for simulation.
However, the circuit shown in FIG. 1 suffers from the following drawbacks.
Since bias currents from two transistors, i.e., 5a and 6a, and 5b and 6b are flowed into the load resistors 8a and 8b, respectively, tradeoff (the relationship that when one is established, the other cannot be established) between two differential circuit design occurs, and it is not easy to optimize circuit constants.
Since the gain of the circuit shown in FIG. 1 is determined by gm1.multidot.R.sub.L1, as described above, when this circuit is applied to a variable gain amplifier circuit, it can be realized by controlling gm1 by changing a current value I.sub.1 of the current source circuit. Thus when DC direct coupling is to be realized, DC potentials at the output terminals 2a and 2b are undesirably changed upon control of the current value I.sub.1.
Furthermore, the amplitude of the output signal of the circuit shown in FIG. 1 is determined by R.sub.L1 .multidot.I.sub.1, and may be applied to a limiter amplifier circuit. When the current I.sub.1 of the current source circuit 10 is changed to allow adjustment of the amplitude of the output signal, the DC potentials at the output terminals 2a and 2b are varied in the same manner as in the variable gain amplifier circuit.
Since R.sub.L1 and R.sub.L2 are directly coupled to each other, when a signal large enough to cause a switching operation (logic operation) is input to the input terminals 1a and 1b, the base-collector capacitance is largely changed due to a change in base-collector voltage of the transistors 6a and 6b. Thus, an output waveform S.sub.2 is distorted, as shown in FIG. 2(b). For this reason, the circuit shown in FIG. 1 cannot be applied to a high-speed logic circuit. Note that S.sub.1 denotes an input waveform.
Furthermore, in monolithic amplifier circuits which will suffer from a variation in manufacturing, peaking characteristics tend to vary, and optimal frequency characteristics are not easy to obtain.